Spacecraft employ solar arrays to convert solar energy to the DC current needed to provide the necessary electrical power on-board the spacecraft. Consisting of large numbers of photovoltaic generators arranged in the rows and columns of a matrix on panels joined together into an essentially planar array that covers a wide two-dimensional area, the solar array is oriented toward the sun and converts the incident light into electricity. To ensure that the individual photo-voltaic generators within the array are functional, it is conventional to test the array and measure the performance of the photo-voltaic generators prior to deployment in spacecraft. Any defective photo-voltaic generators found are conveniently replaced. A solar simulator is used for that test.
The solar simulator provides a pulse of light to the array that emulates light from the sun. Ideally, the solar simulator should provide an equal amount of light over the entire surface of the array, that is, uniform illumination. A standard large area pulsed solar simulator ("LAPSS") contains an electronically controlled electrical load that "dumps" a tailored current/voltage pulse, a pulse of defined width, height and waveshape, as may be viewed on an oscilloscope, into an Xenon lamp, which produces a burst of light or, as variously termed, a light pulse. Typically, the Xenon lamp is housed within a metal box and the light generated is emitted through an outlet aperture or light window, as variously termed, formed in the metal box.
The light pulse is essentially uncontrolled in terms of the light wave characteristic, except as governed by basic principles of physics. At a fixed distance from the test plane containing the solar array, the simulator's light pulse is typically designed to be equal to the intensity of the "solar constant" at the average earth distance from the sun, referred to as AMO, a value expressed in units of watts per square meter. Presently available solar simulators are found to deliver light with an acceptable plus or minus two per cent uniformity, regarded as "uniform" in this field, only over a relatively small area, as limited by the power pulse from the LAPSS's lamp bulb and the distance of the light bulb to the test plane.
A typical 2.5 kilowatt Xenon bulb found in the prior designs for the LAPSS's provides a "one sun" AMO equivalent of the requisite uniformity over a maximum area of eight feet by eight feet square, sixty-four square feet, at a distance to the test plane of twenty-five to twenty-eight feet, typically twenty-six feet. LAPSS's are known which achieve uniformity over an area of 10 feet by 10 feet, but require very high energy light pulses. Still another uses a folding parabolic mirror to achieve uniformity in luminance over a six foot by six foot area where the distance of the light source from the test plane is less critical than that required for large solar arrays.
To provide greater amounts of electricity on board the space craft, solar arrays, referred to as very large solar arrays, are being proposed that are greater in size and cover a larger area. In order to test very large solar arrays, a solar simulator must be capable of providing light of the requisite uniform intensity over an area of up to 400 square feet, that is over a square area of twenty feet by twenty feet in dimension. For reasons not relevant to the present invention, it is desired to accomplish that goal without increasing the distance to the test plane and without increasing the power of the Xenon lamp.
Accordingly an object of the present invention is to provide a new source capable of providing uniform illumination over a large area.
Another object is to expand the coverage area of an existing large area pulsed solar simulator and provide a new solar simulator that provides a relatively uniform plane of light over an area of 400 square feet on a test plane twenty-six distant.
An additional object of the invention is to provide a solar simulator capable of producing a uniform 1 AMO intensity field over a greater area than previously attainable, doing so without an increase in the lamp's size or wattage from that used in a prior simulator and at the same distance between the solar array and the simulator as before.
A still further object of the invention is to provide an improved solar simulator of increased coverage that is simple in structure and relatively easy to fabricate, adjust, and test.
And an ancillary object is to provide an illumination source capable of providing a uniform field of light over large planar surfaces and over curved surfaces as well.